1. Field of the Invention
The present invention relates to a radio communication system, a radio communication apparatus, a radio communication method, and a computer program for performing communications among a plurality of radio stations in a local area network (LAN) or a personal area network (PAN) and, in particularly, to a radio communication system, a radio communication apparatus, a radio communication method, and a computer program for constructing a radio network in which each communication station, free from a slave-master station relationship, performs, in an autonomous distributed manner, a random access operation such as carrier sense multiple access (CSMA) based on a carrier detection or media status monitoring.
The present invention also relates to a radio communication system, a radio communication apparatus, a radio communication method, and a computer program for constructing an autonomous distributed radio network without the intervention of a particular control station with nearby radio systems being free from mutual interference in a communication environment having a plurality of available communication channels. More specifically, the present invention relates to a radio communication system, a radio communication apparatus, a radio communication method, and a computer program for constructing a multi-channel autonomous distributed radio network in which each communication station dynamically and autonomously switches the communication channel in response to the usage status of the communication channel while workload on the network is minimized.
2. Description of the Related Art
Radio networks draw attention as a communication system that frees users from cable connection between devices. Since the radio network eliminates most of cable connection in office work space, communication terminals such as personal computers are relatively easily moved around in an office. With a high-speed and low-cost radio local-area network (LAN) system, the demand for such a system is mounting. A plurality of electronics are set up around individuals as a small scale radio network, namely, a personal area network (PAN). For example, frequency bands, such as a 2.4 GHz band and a 5 GHz band, each requiring no government license, may be specified for radio communication systems and radio communication apparatuses.
Typical standards of radio networks includes IEEE (the Institute of Electrical and Electronics Engineers) 802.11 (see International Standard ISO/IEC 8802-11:1999(E) ANSI/IEEE Std 802.11, 1999 Edition, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications), and Hiper LAN/2 (see ETSI Standard ETSITS101761-1V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1; Basic Data Transport Functions), and ETSI TS101761-2V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 2: Radio Link Control (RLC) sublayer), IEEE802.15.3, and Bluetooth communication. Standards IEEE802.11 include various radio communication methods specified by IEEE802.11a, IEEE802.11b, etc., depending on radio communication method and frequency band in use.
To form a local area network based on radio communication techniques, a control station called an access point or a coordinator is typically arranged to generally control the network.
When a communication apparatus transmits information in a radio network having an access point, an access control method is widely used based on a reserved frequency band. In the access control method, a frequency band required to transmit information is reserved at the access point to use a transmission line that does not collide with information transmission of another communication apparatus. By arranging the access point, a synchronous radio communication is established to keep communication apparatuses synchronous with each other in a radio network.
If a transmitting communication apparatus and a receiving communication apparatus performs a non-synchronous communication in the radio communication system having the access point, the two apparatus must communicate with each other via the access point. This requirement reduces the usage efficiency of the transmission line.
An ad-hoc communication has been proposed as another technique for forming a radio network. In the ad-hoc communication, terminals directly communicate with each other in a non-synchronous manner. Particularly, the ad-hoc communication is appropriate as a small-scale radio network composed of a relatively small number of nearby clients, because terminals directly communicate with each other in a random fashion not via a particular access point in the ad-hoc communication.
The ad-hoc radio communication system, without central control station, is appropriate for use as a home network of home electronics. Even if one electronic apparatus is switched off or fails in the ad-hoc network, routing is automatically updated. The network is thus robust to damage, and data is transferred to relatively far distant location with a high data rate maintained by hopping a packet by several times among mobile terminals. A variety of ad-hoc systems are known as disclosed in the book entitled “Ad Hoc Mobile Wireless Network” authored by C. K. Tho, Prentice Hall PTR.
In an IEEE802.11 based radio LAN system, ad-hoc mobile terminals operating in an autonomous distributed peer-to-peer manner is available without the need for a control station. The IEEE802.11 networking is based on a concept of BSS (basic service set) and divided into two types. In one type, the BSS is defined by an infrastructure mode having a master such as an access point (AP). In the other type, and independent BSS (IBSS) is used and defined by an ad-hoc mode operating with a plurality of mobile terminals (MTs).
The infrastructure mode BSS requires an AP establishing coordination in a radio communication system. The AP integrates an coverage area surrounding own station into a BSS, which is also referred to as a “cell” in a cellular system. An MT near the AP is accommodated into a network as a BSS member. More specifically, the AP transmits a control signal called a beacon each time a target beacon transmit time (TBTT) comes at appropriate intervals. An MT that can receive the beacon recognizes the AP present nearby, and establishes connection with the AP. The MT near the AP receives the beacon, and is ready to recognize a next beacon transmit time by decoding an internal TBTT field. If it is not necessary to receive the beacon each time, the MT can enter a sleep state with power of a receiver switched off for the next TBTT or next several TBTTs.
In the ad-hoc mode IBSS, a plurality of MTs autonomously defines IBSS subsequent to negotiation. Once the IBSS is defined, the plurality of MTs determine TBTT at regular intervals. Upon recognizing the arrival of TBTT referencing own clock, each MT transmits a beacon if the MT recognizes after a random time of delay that no other MTs have transmitted the beacon. Any MT belonging to the IBSS transmits the beacon each time the TBTT comes.
The ad-hoc radio LAN network is known to be subject to the problem of hidden terminals. One station may be exposed to another station, but may be hidden from a third station. Two hidden stations are not able to negotiate with each other, and transmission operations thereof can collide with each other.
CSMA (carrier sense multiple access)/CA (collision avoidance) combined with an RTS/CTS procedure is known to overcome the hidden terminal problem. Standard IEEE802.11 also adopts this method.
In the CSMA (carrier sense multiple access), multi-access is performed based on carrier detection results. In radio communication, it is difficult for own apparatus to receive a signal from own apparatus. To avoid collision in SCMA/CA (collision avoidance) rather than in SCMA/CD (collision detection), own apparatus starts transmitting own information after checking that no information is transmitted from the other communication apparatuses. The CSMA is appropriate for non-synchronous data communication such as a file transfer or an electronic mail.
Carrier detection cannot be performed in an ultra-wide band (UWB) communication, in which one of an extremely short pulse wave having a pulsewidth of one nanosecond or shorter, a signal spread over a band width of several giga-hertz, and a multi-carrier signal is used instead of a carrier in a very wide frequency band. By causing a communication station transmitting data to detect clear status of media, the same random access is performed.
In the RTS/CTS method, a communication station as a data source transmits a transmission request packet RTS (request to send), and starts transmitting data in response to the reception of a notification packet CTS (clear to send) from a communication station as a data destination. Upon receiving at least one of RTS and CTS, a hidden communication station avoids collision by setting, on own station, a transmission suspension period throughout which data transmission is expected in accordance with the RTS/CTS procedure. The hidden communication station, if viewed from a transmitting communication station, sets a transmission suspension period in response to the reception of a CTS to avoid collision with a data packet, and, if viewed from a receiving communication station, stops a transmission period in response to a RTS to avoid collision with an ACK (acknowledgement).
As described above, a network can be established in the ad-hoc mode without any intervention of a control station. But the ad-hoc mode is based on the premise that an IBSS is constructed of a plurality of MTs mutually communicable with each other. A plurality of networks independently operating using the same communication channel are subject to beacon collision or packet collision if communication coverage areas of the networks overlap each other subsequent to the shifting of one network or removal of a blocking object between networks, for example.
FIG. 15 illustrates networks independently operation in IBSS0 and IBSS1 modes, each network composed of three MTs. Even if these two networks use the same channel, no problem is expected because the radio wave of one network is unable to reach the other network. But if the communication coverage areas of two stations STA1 and STA2 overlap each other as a result of shifting of one network IBSS1 as shown in FIG. 16, the packets of the two MTs collide with each other. It is almost impossible to overcome this problem in the radio network having a single communication channel.
The multi-channel communication having a plurality of communication channels is thus introduced. When communication is interrupted by another system or when the spacing in the band becomes scarce because of an increase in the number of participating stations, a communication operation can be started by selecting a communication channel to be used. The network is thus continuously operated together with the other network in coexistence.
The multi-channel communication scheme is adopted in a high-speed radio PAN system of IEEE802.15.3. More specifically, in the adopted algorithm, a radio communication device scans a plurality of usable channels subsequent to power on, recognizes the presence or absence of a device transmitting a beacon signal as a piconet coordinator (PNC) therearound, and selects a frequency channel to be used.
As shown in FIG. 16, the IBSS1 now dynamically switches to a communication channel different from that of the IBSS0 in the network overlapping as shown in FIG. 16. Stations ST4 and ST3 are forced to switch to another communication channel although the two stations are free from interference from the MTs belonging to the IBSS0. More specifically, part of the network under interference causes the entire network to be channel switched. Negotiation involved in the channel switching increases, leading to a reduced throughput.
In the autonomous distributed type ad-hoc network having no control station, resource management of the communication channels is important to minimize mutual interference between radio networks operating nearby. To switch the frequency channel used in the entire network at a time, a representative station referred to as a coordinator or an access point needs to issue, to each terminal, a command indicating a channel in use. After all, it is difficult to switch the frequency channel in the ad-hoc network.
In the IEEE802.11 based LAN system, a network is formed using a frequency channel set by the access point. It is difficult to set up an ad-hoc network without arranging a base station. To communicate with a radio communication apparatus (terminal) belonging to an AP operating at another frequency channel, APs must be connected via a wired LAN cable, for example. Without any connection between the APs, the radio communication apparatuses (terminals) physically adjacent to each other, belonging to different APs, cannot communicate with each other.
In the IEEE802.15.3 high-speed radio PAN system, all frequency channels can be first scanned to search for a coordinator present nearby. Once an operation starts at a particular frequency channel, the usage status of another frequency channel cannot be learned. Even if a piconet at a different frequency channel is present nearby, communications with a radio communication apparatus connected to that piconet cannot be performed.
In the Bluetooth communication, a central control station called a master performs a frequency hopping operation in a random fashion, thereby evenly using frequency channels. The master serves as a standard for a hopping pattern of the frequency channels and synchronization of in time axis, and the presence of the master is essential in the construction of the network. If the master is missing, the network thus constructed is blocked, and a new master must be selected.
The known radio communication systems thus require complex mechanisms including timing for switching the frequency channel, a setup process, such as message exchanging, for participating terminals to start the frequency channel switching in synchronization with each other, and an arbitration process for frequency switching. To carry out self-contained control, the presence of the central control station such as the AP in the IEEE802.11 and HiperLAN/2 communications, and the master in the Bluetooth communication is required. If the central control station, such as the AP or the master, is missing, a protocol or an artificial update process for selecting a new central control station is required. Communications are suspended during such a process.
Network construction methods using a distributed beacon are disclosed as a step to overcome the problem of packet collision in response to a shifting network in Japanese Unexamined Patent Application Publication No. 2003-26457, for example.
In the autonomous distributed radio communication system free from a slave-master station relationship, each station transmits beacon information to notify other stations present nearby (within the communication coverage area) of the presence of own station while notifying the nearby stations of the network construction. Upon receiving a beacon signal, a communication station emerging in the communication coverage of a given station recognizes that own station enters another communication coverage. By reading information in the beacon, the emerging station recognizes the network and can participate in the network.
The stations, mildly time synchronized with each other at beacon transmission timing, performs transmission control utilizing effectively channel resources by means a transmission (MAC) frame permitting a time division multiple access. Each communication station performs a time-synchronized access such as autonomously reserving a frequency band, and setting a priority use period.
In the autonomous distributed network using a distributed beacon, each MT transmits a beacon with TBTTs unoverlapped. Information relating to beacon time of nearby MTs within the coverage area is described in the beacon so that the stations form a network having a beacon position not overlapping an adjacent MT. In other words, a mild IBSS is constructed with respect to each MT.
FIG. 17 illustrates a network that is autonomously constructed using a distributed beacon. If viewed from station STA1, the area represented by an broken-lined ellipse containing station STA4 is an IBSS, and if viewed from station STA5, the area represented by a dot-and-dash-chain-lined ellipse containing station STA3 is an IBSS.
In this way, the problem of hidden station is alleviated while response to the dynamic change in the network becomes possible. More specifically, only an MT having a beacon position overlapping another MT as a result of network shifting updates the beacon position TBTT of own station, and the problem of collision is thus overcome.
The dynamic network is thus constructed using the distributed beacon. However, the construction of the dynamic network is based on the premise that all MTs use the same channel, because the border of the network is different from MT to MT. To switch to another channel, all MTs must be switched in channel as in the ad-hoc mode of IEEE802.11.
Since all MTs use the same channel, and repeatedly use the same time point (slot) or the same band at predetermined transmission frame intervals, each MT is subject to interference from an MT unknown to own MT. If such interference becomes strong depending on radio-wave status, transmission and reception become impossible because predetermined SINR requirements are not satisfied even with a desired available slot.
Japanese Unexamined Patent Application Publication No. 2004-025506 discloses a technique to overcome this problem. In accordance with the disclosure, transmission power from own station is minimized to a level that satisfies the required SINR at a receiver side (transmission power control), and a signal coverage area is controlled by changing a threshold value for detecting a preamble on the receiver side in order to reduce interference (operation of the coverage area by modifying power control and threshold value). Since all MTs use the same channel, interference reduction efforts are subject to limitation depending on the number of communication stations accommodated in the channel.
In the network using the distributed beacon, one channel is divided into slots, each slot being a minimum unit assuring a frequency band for data transmission. A plurality of slots form a transmission frame period (namely, a super frame). Each MT transmits a beacon at each super frame and selects slots not overlapping any MT within an identified area (namely, to a next MT), as a beacon transmission position. Since the length of the super frame is fixed (in other words, the number of slots within the super frame is fixed), the maximum number of MTs that are allowed to participate in the network is equal to or less than the number of slots in the super frame, because of the minimum number of beacon intervals. Even if unused slots remain in the super frame, a new MT cannot participate in the network if the maximum number of MTs is already reached.
A plurality of networks can use different channels as shown in FIG. 18. Networks IBSS0 and IBSS1 using different channels are free from interference. A new STA6 now moves into the network in this communication environment. The newly participating station STA6 is located as shown in FIG. 19, and desires to communicate with both stations STA1 and STA5.
The station STA6 must participate in one of the networks IBSSs. To this end, the station STA6 participates in and then withdraws from each of the IBSS0 and IBSS1 or all MTs must be switched to the same channel. In the former case, the overhead between stations STA1 and STA6 and stations STA1 and STA6 increases. In the latter case, MTs not directly related must be switched to the new channel, and such an operation is otherwise unnecessary.
In the autonomous distributed network having a large number of MTs, the border of the network is different from MT to MT, and it is difficult to effectively share a plurality of channels in known channel assignment methods.
Japanese Unexamined Patent Application Publication Nos. 2003-281586 and 2003-315280 disclose communication systems that use a beacon transmission channel different from station to station, of the MTs belonging to the same network in a communication environment having a plurality of channels. In these disclosed communication systems, an ad-hoc network with interference level lowered is provided by selecting the transmission channel depending on the interference status of each channel.
In accordance with Japanese Unexamined Patent Application Publication No. 2003-281586, each communication station transmits a beacon in a channel appropriate for reception not overlapping the beacon of another station. A communication station having a priority transmission right subsequent to the beacon transmission shifts to a channel appropriate on the receiver side and starts transmission. Communications are thus performed at optimum interference level on the receiver side.
In accordance with Japanese Unexamined Patent Application Publication No. 2003-315280, each communication station determines an average level of interference each nearby station suffers from, and determines, as a transmission channel, a channel resulting in the lowest average interference level. Interferences of nearby stations having high priority with respect to own station are weighted averaged on a per channel basis. A channel resulting in the lowest level of interference the nearby stations suffer from is selected as a transmission channel. The throughput of the entire system is thus heightened.
Each communication station roams or persons frequently move as a radio wave blocking body. If a radio distributed communication system having a plurality of channels is used in such environments, propagation path varies frequently, leading to variations in a beacon transmission channel and a data communication channel. Each communication station frequently needs to scan beacons transmitted from other communication stations, leading to large communication overhead.
In communications using a plurality of channels, the scan operation must be repeated by the number of channels to scan beacons of the other communication stations. Overhead becomes larger than in the communication system using a single channel.